1. Field of the Invention
This invention relates to a pilot fuel injection valve for an internal combustion engine and more specifically to means for controlling the amount of fuel metering by the valve.
2. Description of the Prior Art
The advantages of fuel injection are well known. The degree with which the advantages are obtained is governed greatly by the accuracy and timing flexibility of the metering valve or valves in an injection system and ultimately by the cost of the metering valves and system for controlling the valves. U.S.patent application Ser. No. 403,308, filed Oct. 3, 1973 now abandoned and Div. thereof Ser. No. 689,391 filed May 24, 1976 and assigned to the assignee of this application, discloses a spool type valve capable of metering very small and accurate pilot and main fuel charges to the cylinders of an engine. The valve of application Ser. No. 403,308 employs the concept of metering fuel only while momentarily defining a continuous passage through the valve by traversing a passage in the spool across an outlet passage in the housing. The spool velocity is independent of engine speed; the velocity is preferably the same for all engine speeds and loads. Further, metering is started and stopped without reversing the spool velocity by completely traversing the passage. The traversing feature allows very small and accurate metering of the fuel charges. The features of the same spool velocities for all engine conditions and not reversing direction of the spool during metering allows the use of simple and inexpensive means to control movement of the spool. Further, since direction of the spool is not reversed during metering, spool actuating forces are maintained relatively low. This improves the wear life of the valve, since high actuating forces adversely effect long wear life.
The valve of application Ser. No. 403,308 discloses a throttle controlled sleeve for varying the cross-sectional area of the traversed housing passage, thereby controlling the amount of fuel metered during the traversing time of the passage. This method of controlling the amount of fuel metered adds cost and complexity to the valve and does not readily provide the feature of abruptly lowering pressure of the fuel in the outlet following metering. This feature is desirable since it prevents drible at the injection nozzle connected to the outlet.